A surface acoustic wave device mainly consists of a piezoelectric substrate made of a piezoelectric single crystal material such as lithium niobate (LiNbO.sub.3), a piezoelectric ceramic material or alternatively a combination of a non-piezoelectric plate and a piezoelectric film deposited thereon, for example, and can convert an electric signal to a surface acoustic wave by means of a transducer provided on the piezoelectric substrate and can propagate the surface acoustic wave along the surface of the substrate. It is now employed as filters and other various electronic parts.
FIG. 1 shows a filter as an example of such electronic parts, in which reference numeral 1 designates a piezoelectric substrate. Reference numeral 2 designates an input transducer consisting of a pair of interdigitating comb-shaped electrodes 2A and 2B. Reference numeral 3 denotes an output transducer consisting of a pair of interdigitating comb-shaped electrodes 3A and 3B. When an electric signal is applied to the input transducer 2, it is converted to a surface acoustic wave and travels along the surface of the piezoelectric substrate 1. When the surface acoustic wave reaches the output transducer 3, it is reconverted to an electric signal and is outputted to a load 5. The combshaped electrodes 2A, 2B and 3A, 3B of the input and output transducers are so-called normalized electrodes wherein each electrode finger width W and each space L between respective adjacent interdigitating electrode fingers are .lambda..sub.0 /4, respectively, where .lambda..sub.0 is the wavelength of the center frequency f.sub.0 of a surface acoustic wave to be employed.
A filter including the input and output transducers with the construction of FIG. 1 is subject to electromechanical conversion loss, and is thereby large in filtering loss, because each of the transducers 2 and 3 operates as a so-called bi-directional transducer so as to propagate surface acoustic waves in both the right and left directions.
To alleviate the drawback, there is proposed a unidirectional or mono-directional transducer which is arranged to propagate a surface acoustic wave in a single direction along the surface of the piezoelectric substrate 1. A known example of the mono-directional transducer is shown in FIG. 2, wherein a 120.degree. (or 90.degree.) phase shifter is provided. Another example is shown in FIG. 3, wherein a reflector is provided.
The transducer of FIG. 2 includes comb-shaped electrodes 6A, 6B and 6C to which a signal source 4 is connected via a 120.degree. phase shifter 7 so as to actuate the respective electrodes 6A, 6B and 6C with a 120.degree. phase difference, respectively, thereby propagating surface acoustic waves in only one direction.
This mono-directional transducer, however, must be formed with two-level crossings 8 by interposing spaces or insulative films between at least two electrodes 6B and 6C, for example, which are different in their phases. This causes a very complex manufacturing process, a worse productivity of the device and an increase of the production cost.
The transducer of FIG. 3 comprises a power supply section and a reflecting section including normalized comb-shaped electrodes 9A and 9B, respectively, and a common electrode 10. The power supply section 9A is connected to the signal source 4 via a matching circuit 11 while the reflecting section 9B is connected to a reactance circuit 12 so that the reflecting section 9B terminating at the reactance circuit 12 reflects leftwardly travelling components of surface acoustic waves which propagate in both the right and left directions from the power supply section 9A, and thereby allows the surface acoustic waves to travel merely in the right direction.
This mono-directional transducer, however, comprises the normalized electrodes 9A and 9B whose electrode finger width W and whose space L between adjacent electrode fingers are each .lambda..sub.0 /4, respectively. Thus, the surface acoustic waves reflected by the respective electrode finger tips of the electrodes 9A and 9B coincide with each other in their phases, thereby enlarging interelectrode reflections. This worsens the nature of the device.
To alleviate the drawback of the transducer employing the reflector of FIG. 3, there is proposed a double-electrode transducer as shown in FIG. 4. In this transducer, each electrode finger of the interdigitating comb-shaped electrodes 2A and 2B is divided into two parts so that each electrode finger width W and each space L between the electrodes is .lambda..sub.0 /8.
This structure certainly reduces influences by reflecting waves because the reflecting waves at the respective electrode finger tips are different in their phases by 180.degree., that is, they are in opposite phases, and counteract each other. However, it requires extremely high accuracy upon making the electrodes because the higher the frequency, the smaller the wavelength .lambda. becomes. If the center frequency is 1 GHz and the piezo-electric substrate is made of lithium noibate, which is most widely adopted, .lambda..sub.0 /4 is 0.87 .mu.m and .lambda..sub.0 /8 is 0.44 .mu.m, approximately. Such extremely small measurement makes it difficult to constantly fabricate the devices with uniform characteristics even if a recent precision working technique is employed.